Filamentary metal brushes have been used to transfer electrical energy from a power source to the rotating member of a machine. The brushes comprise a plurality of fine hair-like metallic fibers that are individually suspended independent of the surrounding fibers. Each of the fibers provides a discrete contact point with a metallic ring surface of the rotating member of the electrical machine. Accordingly, numerous contact points are established and act in concert to conduct electrical current between the brushes and the rotating member of the electrical machine.
As the amount of current has increased in electrical machines, the format of the brushes has changed. In order to handle large amounts of current, the hair-like brushes have been replaced by solid blocks of graphite which provide greater serviceability. Graphite has been used because it provides natural lubrication and vaporizes at a high temperature rather than melts. Furthermore, the formation of a protective film on the ring surface is formed from the presence of atmospheric humidity and oxygen. The protective film shifts wear from the ring to the brush which is easily replaced during routine machine maintenance.
The graphite brushes wear out from both electrical wear and mechanical wear. The electrical wear results from the vaporization of the graphite at scattered contact points of the ring that randomly move across the interface surface. Mechanical wear results from the dry mechanical friction between the graphite and the surface of the metallic ring. In order to ensure proper operation of the electrical motor, inspection and servicing of the graphite brushes are required.
Recently, metal fiber brushes have been incorporated into electrical motor designs. These brushes incorporate numerous metal fibers bound together into a solid block that resembles a standard graphite brush. The fibers are fused or bonded together using a matrix material. However, these brushes cannot handle the high current density required for industrial applications.